Electrical power generation is indispensable to modern society, and ranges in scale from enormous, coal-fired, nuclear and hydro-electric power plants to small electrical generators running on hydrocarbon fuels. In recent years, the prevalence of electrical energy production technologies having a perception of cleaner or more efficient operation has increased. Fuel cells in particular have received increasing attention in both technical and commercial circles.
In one common type of fuel cell power generation system, a fuel cell is provided which outputs electrical current to an electrical grid or an electrical power-driven device, often referred to as the utility. In many circumstances, the output voltage of the fuel cell will be relatively lower than the desired voltage that will ultimately be supplied to the utility. Similarly, because AC power is more widely used than DC power, in many instances it will be necessary to invert a DC output power from the fuel cell to a suitable AC power, for example, 3-phase AC electrical power.
An electronic system commonly known in the art as a “power converter” will typically be disposed between the fuel cell and the utility, which appropriately conditions a DC output from the fuel cell prior to its delivery to the utility. Many power converter designs are known in the art. In one common design a first stage, known as a “boost converter” is primarily responsible for boosting the fuel cell output to a higher voltage, called the DC link voltage. An “inverter” may be coupled with the boost converter via a DC link, and inverts the DC power from the boost converter to AC power.
Any electrical power generation system will have certain operating requirements and challenges that must be addressed for successful operation. For instance, many power sources are sensitive to certain electrical disturbances from components downstream thereof. Ripple currents are one such disturbance and are a well-known problem. If left unchecked or uncontrolled, ripple currents can actually cause or speed degradation of internal components of a fuel cell. Many fuel cell manufacturers recommend particular ripple current limits, or total current to ripple current ratios, for optimal operation and durability. In the past, power generation system designers have typically implemented input filters to reduce ripple currents between the fuel cell and the power converter. Such filters may include reactive components, such as inductors, that tend to be relatively large, heavy and expensive. The size of the inductors used in the input filter is generally inversely proportional to the input power levels expected to be encountered during operation. In other words, where low power levels are expected during operation, as is usually the case at least some of the time, relatively large and expensive input filters must be used if ripple current requirements are to be met.
Other operational requirements are concerned with the actual power transferred between the electrical power generation system and the utility. One example of such requirements are the limits which may be imposed by regulatory agencies on individual harmonic currents and total harmonic distortion produced when an inverter interacts with the utility, absorbing or supplying power. In this instance, harmonic currents may be generally defined as undesired currents having a frequency different from the frequency of the fundamental or desired output current. In many instances, the harmonic currents can generate radio frequency distortion that can interfere with various electronic devices. Engineers have addressed output current harmonics in a manner similar to the approach applied to input ripple currents, namely, with an output filter, however, the output filters tend to have shortcomings similar to the input filters discussed above.
U.S. Pat. No. 6,369,461 to Jungreis et al. discloses one electrical power generation system that includes a fuel cell, an inverter and a boost converter. Jungreis et al. utilize a battery coupled with a DC bus via a diode to support large positive load steps that cannot initially be compensated for with a fuel cell. The boost converter is said to be sized for common load changes rather than maximum possible load changes. While the boost converter may be relatively smaller than in certain other designs, the use of the battery and diode introduce substantial complexity into the system.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.